Devices that filter selected wavelengths of light are well known and have been in use for many years. Typically, a source of white light or light comprising components from wavelengths throughout the visible spectrum is filtered so as to transmit only the desired wavelengths. Among the various filters that are conventionally available for use as a light filter, absorption filters and interference filters are in common use.
One type of interference filter utilizes a dielectric layer disposed between two very thin layers of a reflective material. The resulting filter transmits light within a band of the visible spectrum. The wavelength band of the transmitted light is not constant at all angles of incidence, however. Typically the transmitted band will shift as the angle is changed. The apparent color of transmitted light will therefore change as the viewing angle of an observer is changed. The wavelength of light that is transmitted and the magnitude of the band shift are directly related to the thickness of the dielectric layer and the index of refraction of the dielectric material.
Because there are many dielectric compounds from which to choose, and because the dimensions of the various components can be finely controlled, band pass interference filters can be manufactured to transmit light in a wide variety of bands along the visible light spectrum.
What are needed in the art are novel filters that advantageously use the optical properties of conventional interference filters to produce useful filtering effects.